I don't see why one reading should necessarily exclude the other. For example, I could be an [old book] [collector] or an [old][book collector]. So it seems reasonable to me that we could have a [dark polka dot][tie] or a [dark][polka dot tie]. In both of the examples above book and polka dot are nouns modifying other nouns, so do I not concur that blue must be an adjective to be able to modify necktie in McCawley's example.This would seem to be confusing the word category with its syntactic function, in other words with it grammatical relations within the noun phrase. Having said this I am mute asI'll decline to comment on the word category of blue in the original example, because I am not convinced either way.
It seems reasonable to conclude from the data above that both [dark blue][necktie] and [dark][blue necktie] might be possible. There is, of course, a third possibility, which is that the two modifiers might be in co-ordination so that the structure is the same as would be shown by more explicitly by dark, blue, necktie where the commas would indicate that that the attributes applied independently to the tie.